The present invention relates generally to electronic switches and, more particularly, to an arc suppressing circuit employing a triggerable electronic switch to protect switch contacts.
In systems where power to a load is switched using an electro-mechanical switch, wear of the contacts of the switch often occurs due to sparking or arcing between the contacts of the switch primarily during times of opening and closing of the switch and, more particularly, when the switch contacts xe2x80x9cbouncexe2x80x9d during closing of the switch. Arcing across the contacts arises due to a voltage difference across the contacts of the electrical switch that is caused by the bouncing of the switch contacts. To illustrate an example of circuit conditions occurring during bouncing of an electro-mechanical switch, FIGS. 4 and 5A-5C show a conventional relay switching circuit and the voltage and current conditions occurring in the circuit. The circuit 400 shown in FIG. 4 illustrates a relay switching circuit including a voltage source 402 supplying voltage through a relay switch 404 to a load 406 (e.g., a motor). The relay switch 404 has two contacts 408 and 410, which are electrically connected together when a voltage from source V2 is applied to relay coil 412.
As illustrated in FIG. 5A, a voltage is present across contacts 408 and 410 when the switch 404 is open. At a time t1, the relay coil 412 is energized thereby creating a magnetic field that presents a force to close switch 404. After a time delay from time t1 to time t2, the contacts 408 and 410 of switch 404 are electrically connected together and the voltage across the contacts drops to zero volts as shown in FIG. 5A. Also at time t2 the voltage is delivered to the load 406 and current begins to flow through the load 406 as shown in FIG. 5B. The switch 404, however, tends to bounce, which creates arcing across the contacts of the switch 404 due to a voltage arising due the break of electrical contact. This voltage rise due to bouncing of the switch 404 is illustrated in FIG. 5A between time t2 and time t3. It is this voltage rise and associated arcing that causes wear to the contacts of the electrical switch.
One approach to mitigate the effects of arcing in power control circuits that have need for relay switching (e.g., motor controllers) is to use solid state relays since their life exceeds that of conventional electro-mechanical relays. Electro-mechanical relays are shorter lived due to the arcing explained above. Solid state relays, however, are much more costly than conventional electro-mechanical relays and require heat sinking, which increases the space required for the solid state relay. In cases where the cost or size of solid state relays is prohibitive, substitution is usually made by providing a larger and higher rated electro-mechanical relay so as to increase the life of the relay contacts in a particular circuit. This, however, also increases the cost and size requirements for the electro-mechanical relay switching.
Another approach to mitigating contact wear, is to employ arc suppression circuits that prevent or extinguish arcing by shorting in parallel with a switch during periods of arcing, thereby increasing the switch life. Some known arc suppressing circuits include a triggerable electronic switch, such as a triac, in parallel with a switch. In such circuits, the triac is typically triggered by a triggering circuit that senses when voltage is present across the contacts or triggers during known periods of contact opening, closing or bouncing. Such triggering circuits can be complex and add components to the switching circuitry, which increases cost and complexity of the circuit. Additionally, the circuits typically require heat sinking of the triac semiconductor due to the triac conducting for a number of AC cycles, which increases the space needed for the arc suppression circuitry.